Kalirin, a large, multi-domain Rho GDP/GTP exchange factor (GEF), is expressed both within and outside of the nervous system. Genetic studies revealed roles for human KALRN in early onset coronary artery disease, schizophrenia, Alzheimer disease and attention deficit hyperactivity disorder. The use of several promoters and alternative splicing generate multiple isoforms of Kalirin. Studies to date have focused on Kal7, the major isoform in the adult brain, revealing an essential role for its ability to activate Rac1 n dendritic spine formation and function. Kal9 and Kal12, the major isoforms expressed outside of the nervous system, include a second Rho GEF domain specific for RhoA. Mice unable to produce any of the major isoforms of Kalirin (KalSRKO) share the deficits observed in Kal7KO mice and exhibit an array of additional deficits. Most prominent is their stunted growth, impaired maternal behavior, reduced atherosclerosis, diminished trabecular and cortical bone mass and impaired neuromuscular junction formation. We first identified Kalirin based on its interaction with an essential secretory granule membrane enzyme, peptidylglycine ?-amidating monooxygenase (PAM). Our analysis of KalSRKO mice and expression of Kalirin in a corticotrope cell line support the hypothesis that Kalirin plays a crucial role in the ability of peptide secreting cells to adjust their secretory pathway to environmental inputs such as growth factors and extracellular matrix components. In Aim 1, we will use corticotrope tumor cells as our bioassay system for understanding how the Sec14 domain of Kalirin affects secretion and endocytic trafficking. Using a FRET biosensor for activated Rac1, we will relate Rac1 activation to exocytosis. Key findings will be tested using primary pituitary cultures from wildtype and KalSRKO mice. In Aim 2, we will use a structural approach to build towards understanding how the presence of a Sec14 domain, multiple spectrin repeats and a Rho GEF in a single protein is used to accomplish the precise spatial and temporal control needed to regulate membrane trafficking. Purified recombinant Sec14, spectrin repeat regions and Kal7 will be characterized with in vitro assays and used for crystallographic studies. Secreted peptides and proteins have widespread modulatory effects on cell growth and differentiation. A better understanding of the mechanisms by which Kalirin conveys signals to the pathways through which basal and regulated secretion occur will offer specific targets for intervention and control of secretion.